Strip lapacke.h to only the declarations used by Eigen

Summary

• Remove 948 unused LAPACKE function declarations from Eigen/src/misc/lapacke.h
• Keep only the 32 declarations (8 routines × 4 type variants) Eigen actually calls: potrf, getrf, geqrf, geqp3, gees, gesdd, gesvd, syev/heev
• Preserve all required type definitions, constants, callback typedefs, and name mangling macros
• 10,085 lines (860KB) → 218 lines (~8KB) — 97.8% reduction

This eliminates a major source of conflicts with system LAPACK headers (non-standard const qualifiers, missing parameters) reported in the issue.

Test plan

Local Docker (GCC 15, CMake 3.31, Debian trixie, C++17):

Test suite	Tests	Result
eigen_blas	library build	✅ links OK
eigen_lapack	shared library build	✅ links OK
cholesky	6 variants	✅ 6/6 pass
lu	2 variants	✅ 2/2 pass
inverse	5 variants	✅ 5/5 pass
qr	7 variants	✅ 7/7 pass
eigensolver_selfadjoint	13 variants	✅ 13/13 pass
eigensolver_generic	5 variants	✅ 5/5 pass
product_large	9 variants	✅ 9/9 pass
Total	42 tests + 2 library builds	42/42 pass

Closes #2851 (closed)